Check out Gtk2::TrayIcon - but I'm not sure if that is a standards complient implementation. Otherwise the specs don't seem to be too complicated, so you might write your own implementation.
I've never messed with tray icons before, but I have used GTK+ (python bindings...not my choice). It seems that the tray icon runs as it's own separate program (runs the GTK main loop). How would I make this work as an irssi plugin?